Loop-around testing permits individual loops to be tested end-to-end by placing a call to the common central office via a first line to access a loop-around test control unit. The number of the line to be tested is then dialed from the remote location into the control unit, and a connection is set up through the control unit which includes both the call-in loop and the loop to be tested. Normally, a craftsperson is present at the far end of the line under test.
The connecting set in the central office for earlier loop-around test control circuits, passed voice frequency signals. A potential thus existed for fraudulent use of the network by those who have knowledge of how to access the central office loop-around test equipment. In response to this, more recently designed loop-around test equipment provided for the passing only of a tone signal and the blocking of any speech signals. Unfortunately, however, the noise occasionally present on some carrier system looks like speech signal; and hence the system would disconnect at undesired times as well as when intended to.